The present invention relates generally to cell reselection in a cellular, or other radio, communication system. More particularly, the present invention relates to apparatus, and an associated method, for selectively permitting initiation of effectuation of cellular reselection, such as a communication handover, or other control-channel-intensive event, of a mobile station operable in the communication system. A prediction is made of the likelihood that the cell reselection shall be successful. If the predicted likelihood is less than a minimum threshold, a request is made to increase the diversity, e.g., the coding rate, of control signals used in the cell reselection process. Cell reselection is permitted only when the likelihood of success is predicted to be high. Thereby, the possibility of loss of communications during the cell reselection is reduced.
Advancements in communication technologies have permitted the implementation, and widespread usage, of multi-user radio communication systems. A cellular communication system is exemplary of such a radio communication system. Information signals generated during operation of the radio communication system are transmitted upon radio communication channels defined upon portions of the electromagnetic spectrum. Regulatory bodies allocate portions of the electromagnetic spectrum for communications in the various communication systems.
A cellular communication system includes network infrastructure which is installed throughout a geographical area which is to be encompassed by the system. The network infrastructure includes a plurality of spaced-apart radio base stations located at spaced positions throughout the geographical area. Groups of the radio base stations are coupled to a base station controller which operates to control various functions of the radio base stations. And, groups of base station controllers are coupled to mobile switching centers which perform switching operations to permit the performance of communication operations.
Each of the radio base stations defines a cell. A cell is a portion of the geographical area encompassed by the cellular communication system and positioned proximate to a radio base station. Generally, a mobile station positioned in a cell defined by a radio base station is able to communicate with such radio base station.
Cellular communication systems efficiently utilize the frequency bandwidth allocated for communications therein. Because of the positioning of the radio base stations at spaced positions throughout a geographical area, only relatively low-power signals need to be generated by the mobile station and also the radio base station to effectuate communications therebetween. The same channels can be reused throughout the cellular communication system according to a cell reuse plan. Because of such reuse of the same channels, the same channels can be concurrently used in different ones of the cells defined by the communication system.
However, because only relatively low-power signals are generated, as a mobile station travels throughout the geographical area encompassed by the cellular communication system, the mobile station cannot continue communicating with the same radio base station. That is to say, as a mobile station passes from cell-to-cell, the mobile station must communicate with successive ones of the radio base stations. Thereby, communications with the mobile station can continue as the mobile station passes through the successive cells.
In some cellular systems, the transfer of communications with the mobile station from one radio base station to another base station is referred to as a hand-off, or handover, of communications. The radio base station with which the mobile station has ongoing communications is sometimes referred to as a serving-cell and the radio base station with which a hand-off of communications is to be effectuated is sometimes referred to as a target cell. Handover of communications, or more generally, cell reselection, is a fairly complex process requiring, variously, a determination that the handover should occur, requests for, and allocation of, channel capacity at the target cell to continue communications, and, thereafter, effectuation of the handover of communications. Other control-channel-intensive events are analogously carried out as a mobile station passes from one cell to another.
In various cellular communication systems, the mobile station must be retuned from channels used at the serving-cell to channels used by the target-cell, requiring, thereby, action by both the network infrastructure and the mobile station to effectuate the handover. And, in various other systems, such as a CDMA (code-division, multiple-access) communication system, such as that defined in the IS-95 specification promulgated by the EIA/TIA, a process referred to as a soft handover is effectuated. In a soft handover, handover of communications from a serving-cell to a target-cell is effectuated without the need of the mobile station to retune to a different channel.
In other types of communication systems, analogous procedures are performed. For instance, in GPRS (general packet radio service) communications defined in a GSM (global system for mobile communications), a cell reselection procedure is effectuated in which communications between the mobile station and the serving-cell are terminated, and, thereafter, communications are restarted with the target-cell.
Handover of communications from a serving-cell to a target-cell are intended to be transparent to the use of a mobile station. That is to say, effectuation of handover of communications occurs without detection by a user. But, because a handover procedure is a relatively complex procedure, errors which might occur during the effectuation of such procedure might result in a xe2x80x9cdropped callxe2x80x9d or other disturbance which interferes with ongoing communications.
A manner by which to reduce the possibility that an error occurring during a control-channel-intensive event, such as handover of communications, or other cell reselection, from a serving-cell to a target-cell which might result in disruption of ongoing communications would advantageously improve operation of, and a user""s perception of, communications in the radio communication system.
It is in light of this background information related to cell reselection in a radio communication system that the significant improvements of the present invention have evolved.
The present invention, accordingly, advantageously provides apparatus, and an associated method, for selectively initiating effectuation of cell reselection, or other control-channel-intensive event, in a radio communication system. Cell reselection is permitted only if an acceptable likelihood of success of effectuation of the cell reselection is predicted. By permitting effectuation of cell reselection only if there is an acceptable likelihood that the effectuation of the cell reselection shall be successful, the possibility of a dropped call, or other undesired termination of communication, is reduced.
In one implementation, prediction is made of the likelihood of successful effectuation of the cell reselection, such as a communication handover, of a mobile station operable in a cellular communication system. If the predicted likelihood is less than a minimum threshold, effectuation of the cell reselection is not permitted. Instead, a request is made to increase the diversity of control signals transmitted between network infrastructure of the cellular communication system and the mobile station and used during cell reselection procedures. By increasing the diversity of the control signals, the likelihood that such control signals shall be accurately detected and acted upon is increased. Thereby, subsequent cell reselection is more likely to be effectuated successfully.
In one implementation, predictions are made dynamically, that is to say, subsequent to increasing the diversity of the control signals, and prediction is again made as to the likelihood of successful effectuation of cell reselection. If the predicted likelihood of successful effectuation is of at least an acceptable level, cell reselection is permitted.
In one implementation of the present invention, a mobile station is operable in a cellular communication system and is permitted to travel between cells defined in the communication system. Conventional signal monitoring techniques are carried out, to determine whether handover of communications from a serving-cell to a target-cell would be possible or appropriate. Responsive to such indications, measurements are made of the average SNR (signal-to-noise radio) of selected control signals generated during operation of the cellular communication system. When the cellular communication system is a GSM (global system for mobile communications) system, the control signals include those transmitted upon a BCCH channel. Responsive to such determinations, a likelihood of successful effectuation of handover from a serving-cell to a target-cell is predicted. If the predicted likelihood is at least an acceptable level, handover of communications is permitted. If the predicted likelihood is less than an acceptable level, a request is made to increase the diversity of the control signals, thereby to increase the possibility that the control signals are accurately detected.
In one implementation the control signals are coded at a selected coding rate. When a request is made to increase the diversity of the control signals, the coding rate at which the control signals are coded is increased.
In a further implementation, RLP (radio-link protocol) signaling is utilized in which ARQ response messages are generated responsive to detection of RLP messages. The probability that all of the messaging required of a handover exchange is successful is computed. The computed probability forms the prediction of the likelihood that effectuation of the handover shall be successful. If the computed probability is less than a minimum level, a request is made to increase the coding rate at which control signals are encoded. If, conversely, the computed probability is greater than the threshold probability level, effectuation of the handover is permitted.
In one implementation, when a potential for handover of communications from a serving-cell to a target-cell exists, determined e.g., through a conventional neighbor cell list signal energy measurements, signal-to-noise ratio measurements of control signals communicated between the serving-cell and mobile station, and also between the target-cell and the mobile station, are made. Mean values of the signal-to-noise ratios are determined as is also a standard deviation of the signal-to-noise ratios. From such determinations, a density of the block error rate, at a specific channel coder rate, is determined. Then, message delays based upon correlated, or independent, message failures are modeled. And, the probability that an ARQ process of RLP signaling used in handover procedures shall terminate without completing successful message transmission is computed. From the computed probability, a probability is computed that the entire handover exchange is successful. This probability is compared against a threshold value. If the computed probability is less than the threshold value, a request is made to increase the coding rate of the control signals. Otherwise, the likelihood of successful effectuation of handover is at least an acceptable level, and effectuation of the handover is permitted.
In one embodiment, a mobility manager, forming a portion of the network infrastructure of a cellular communication system is operable to make the determinations and computations required to predict the likelihood of successful effectuation of the handover. In various implementations, the mobility manager forms part of a base station controller, mobile switching center, or SGSN of the network infrastructure of the communication system.
In these and other aspects, therefore, a method, and an associated apparatus, selectively initiates effectuation of a control-channel-intensive event for communications by a mobile station in a radio communication system. The radio communication system has a serving-cell, fixed-site radio transceiver and at least one target-cell fixed-site radio transceiver. The serving-cell, fixed-site transceiver defines a serving-cell, and the at least one target-cell, fixed-site transceive defines a target-cell. A parameter associated with a signal-to-noise ratio of selected control signals generated by the serving-cell, fixed-site radio transceiver are determined. Responsive to values of the determined parameters, a likelihood of successful effectuation of the control-channel-intensive event is predicted. If the likelihood of successful effectuation is predicted to be less than a minimum selected threshold, the selected control signal generated by at least the target-cell, fixed-site radio transceiver are requested to be of increased diversity levels. Conversely, if the likelihood of successful effectuation of the control-channel-intensive event is predicted to be of at least an acceptable threshold, initiation of the effectuation of the cell reselection is permitted.
A more complete appreciation of the present invention and the scope thereof can be obtained from the accompanying drawings which are briefly summarized below, the following detailed description of the presently-preferred embodiments of the present invention, and the appended claims.